Drilling fluids perform a variety of functions that influence the drilling rate, the cost and safety of a well drilling operation. The drilling fluid, or so called mud, is pumped down a hollow drill string, through nozzles in the bit at the bottom, and then up the annulus formed by the hole or casing and the drill string to the surface. The bit is turned by rotating the entire string from the surface or by using a downhole motor. Circulation of the drilling fluid in this manner removes the cuttings from the bore hole, lubricates and cools the drill bit, seals the wall of the bore hole with a thin impervious layer and applies a hydrostatic head to the formation to counterbalance formation pressures. After reaching the surface, the drilling fluid is passed through a series of screens, settling tanks or other equipment to remove formation material brought to the surface. It is then treated with additives to obtain a desired set of properties. Once treated the fluid is pumped back into the well and the cycle repeated.
Most drilling, completion and workover fluids are complex mixtures of interacting chemical compounds and their properties change remarkably with changes in temperature, shear rate and shear history. As drilling fluids circulate through the bore hole they are subjected to ever changing conditions such as turbulent flow in the drill pipe, intense shearing at the bit, and nominal laminar flow in the annulus.
Accordingly one of the most desirable characteristics of drilling fluids as a class of material is versatility which can be achieved by foresighted treatment with carefully selective additives. A variety of drilling fluid additives are known. The commonly used additives include clays, viscosifiers, fluid loss control agents, thinners, lubricants, biocides, surface active agents, weighting materials, flocculants, shale inhibitors, defoamers, caustic, salts, etc. In drilling certain earth formations such as gypsum or other soluble organic salts, or formations containing swelling clays, or also in deep hot wells, a gelatin and/or thickening of the drilling fluid frequently occurs, such that the viscosity of the drilling fluid becomes excessive and there is danger of loosing control of mud properties, of high pump pressure, of slowing the drilling rate, of not passing through screen of shale shaker, of loosing mud circulation, of gas cutting in the mud, or of blowout occurring. For control of rheological flow properties of the drilling fluid, which is the primary concern of this invention, knowledge of the plastic viscosity (PV), yield point (YP), and the gel strengths (Gel) is necessary. The PV/YP ratio characterizes the shear thinning properties of the drilling fluid i.e. the degree to which the effective viscosity declines with increase in rate of shear. Excessively high yield points or gel strengths are reduced by the addition of certain compounds known as thinners. When added to the drilling fluid these thinners reduce flow resistance and gel development. Materials commonly used as thinners for clay-water drilling fluids can be broadly classified as (1) plant tannins, (2) polyphosphates, (3) lignite materials, (4) lignosulfonates, and (5) synthetic polymers.
Numerous drilling fluid additive mixtures comprising various combinations of materials such as lignosulfonates, modified lignosulfonates, lignite, modified lignite, tannins and modified tannins with selected metal compounds [lave been patented. Reference is had to U.S. Pat. Nos. 4,704,214, 4,842,770 and 4,618,433 the disclosures of which are incorporated herein by reference.
While the additive mixtures disclosed in the above mentioned patents have been successfully employed in well drilling operations there is a continuing need to provide a more cost effective additive which is environmentally acceptable.
Accordingly, it is an object of this invention to provide new drilling fluid additives having improved thinning properties.
A more specific object of this invention is to provide an improved drilling fluid having enhanced viscosity or other rheological characteristics.
Another object of this invention is to provide a method of using the improved drilling fluid in the drilling, completion, or workover of wells.
Still another objective of this invention is to provide a method of producing drilling fluid additive blends which are characterized by increased simplicity and efficiency.
Other aspects, objects and advantages of the invention will be apparent to those skilled in the art in view of this disclosure.